The conventional photoelectric conversion elements such as photodiodes (PDs) and avalanche photodiodes (APDs) are fabricated by forming pn-junction semiconductor layers on a high-density p-type or n-type semiconductor substrate by crystal growth, impurity diffusion, and so on. In recent years, in terms of high photosensitivity and high response, semiconductor photodetectors in which electrodes containing bonding pads for electrical connection with electrodes are provided on a semi-insulating substrate have been developed and are in practical use.
For example, Japanese Patent Application Laid-Open No. 63-285971 (Document 1) discloses a semiconductor photodetector in which two types of semiconductor layers with different characteristics are exposed on an identical plane of a semi-insulating substrate to enable placement of electrodes on a flat surface. The semiconductor photodetector of this type was useful in terms of achievement of low capacitance, but required special techniques of forming depression portions of two steps with a small depth difference in a flat portion of the semi-insulating substrate and etching back the semiconductor layers deposited in the depression portions and a substance with different characteristics, such as a photoresist, at almost equal rates in its production process.
Disclosed as a technique for remedying the problem as described above is a semiconductor photodetector which has two types of semiconductor layers crystal-grown in a mesa shape on a flat surface of a semi-insulating substrate and in which electrodes are laid on exposed portions of the semiconductor layers. The semiconductor photodetector of this type is disclosed, for example, as a conventional example in Japanese Patent Application Laid-Open No. 2-105584 (Document 2).